This invention relates to video signal recorders and, more particularly, to a method and apparatus for cancelling the remanent deflection which is present in the piezoceramic head-support device of such a recorder.
Recently, piezoceramic support assemblies, such as a monomorph support leaf and a bimorph support leaf, have been designed to support the scanning transducer, or head, in a video signal recorder, such as a video tape recorder (VTR). The piezoceramic support device is responsive to a control voltage applied thereacross to deflect in a direction and by an amount which is determined by the polarity and magnitude of the control voltage supplied thereto. Thus, during a reproducing mode of the VTR, for example, if the head which then is scanning a previously recorded record track deviates from that track, the piezoceramic support device is controlled to deflect by a specified amount so that the head which is supported thereby is displaced by the same amount and, hence, is brought into proper tracking alignment with the record track which it is scanning. By properly controlling the piezoceramic support device, mis-tracking errors in a normal reproducing mode, as well as in various special reproducing modes, such as the stop-motion mode, the slow-motion mode, the quick-motion mode, and the like, can be minimized. As a consequence of minimizing such mis-tracking errors, the video picture which ultimately is reproduced is of desirable quality. An example of VTR systems in which a piezoceramic support device is used is disclosed in U.S. Pat. Nos. 4,165,521 and 4,163,994. Examples of the material which can be used to construct the piezoceramic support device are described in Journal of Applied Physics, Volume 46, No. 1, January 1975, pages 222-229; and in the paper "Effects of Ageing and Compressive Stress on the Properties of BaTiO.sub.3 Ceramics" by N. J. Poole, Journal of Physics (Great Britain), Volume 8, 1975.
When a piezoceramic support device, such as a bimorph leaf assembly constructed of two piezoceramic leaves whose directions of polarization are opposite to each other, is supplied with a control voltage of given polarity, the piezoceramic support device bends, or deflects, in a direction which is determined by that polarity. If the polarity of the control voltage is reversed, the direction of deflection likewise is reversed. During a reproducing operation, it is expected that a tracking control voltage of changing magnitude and polarity will be applied to the piezoceramic support device to correct for mis-tracking errors. It has been found that the piezoceramic material exhibits hysteresis. That is, as the tracking control voltage increases, a first amount of deflection will be produced in response to a tracking control voltage of a predetermined level, but then, as the tracking control voltage is reduced, when that level of voltage once again is reached, a second, greater (or lesser) amount of deflection will be obtained therefor. Because of this hysteresis, a residual, or remanent deflection, will be present even if the tracking control voltage is reduced to zero. This means that, when the tracking control voltage is removed from the piezoceramic support device, the head which is supported thereon will not return to its "home" position.
In view of the foregoing phenomenon of remanent deflection, referred to hereinafter merely as remanence, a mis-tracking error may result during a reproducing operation because of the failure of the piezoceramic support device to respond to the tracking control voltage supplied thereto in the manner for which the tracking control system had been designed. For example, if the VTR is operated during an edit operation, previously recorded video signals first will be played back, and then new, additional information will be recorded. Because of the remanence of the piezoceramic support device, the pitch between the last track which is played back and the first track which is recorded may be either too large or too small. This discrepancy is attributed to the fact that, during the recording section of the edit operation, no tracking control voltage is applied to the piezoceramic support device, and the head supported thereon is expected to return to its home position for recording. The discontinuity between the previously recorded and newly recorded tracks is due to the failure of the head to return to its home position. As another example, if the VTR is a so-called two-head recorder, and if it is operated in the edit mode, as before, then one piezoceramic support device may be deflected in one direction, for example, in the upward direction, while the other piezoceramic support device is deflected in the opposite direction in order to correct for mis-tracking errors during the reproducing mode of the edit operation. If, after the last track is played back, the VTR is changed over to its recording mode wherein no tracking control voltage is supplied to the piezoceramic support devices, the remanence of these devices will result in one head being displaced upward from its home position and the other head being displaced downward from its home position. The pitch of the tracks which then are recorded by such heads will be non-uniform, wherein the spacing between adjacent tracks will be greater-than-normal, followed by less-than-normal, followed by greater than-normal, and so on.